There are many burglar alarm systems presently available on the market. Some of the oldest of such systems use point-to point wiring, both for opening detection purposes, and for operating the alarms responsive to such detection. Such point-to-point wiring is costly and cosmetically defacing.
Some of the newer burglar alarm systems, therefore, use entranceway opening detection devices that transmit radio frequency signals to a remote receiver which enables an alarm.
The radio frequency-utilizing burglar alarm systems also have certain definite drawbacks. For example, these detecting devices, generally, are battery powered to enable the generation of radio frequency signals responsive to unauthorized openings of the entranceways. Batteries have limited life spans; and therefore, require either special circuitry for automatically checking the power output of the batteries, or periodic checks have to be manually accomplished. Indeed, battery checking and maintenance all too often are neglected with such devices resulting in their uselessness. Such battery operated devices, for the most part, operate through a constantly generated signal while the doors and windows are open which even more so wears the batteries.
In addition, the receiving devices, being in the radio spectrum, are inherently susceptible and operation responsive to spurious radio signals that are either of the same frequency or subharmonics and harmonics of the operating frequency; and therefore, the prior art radio frequency utilizing burglar alarm systems are prone to give "false alarms."
Many attempts have been made to eliminate the "false alarm" problem. The "false alarms" tend to cancel the effectiveness of a burglar alarm system, while presenting the potential for a needless traumatic experience.
Accordingly, an object of the present invention is to provide new and unique burglar alarm systems which are practically immune to "false alarm" due to spurious signals. "Spurious signals" in this application means signals which are generated by means not associated with the burglar alarm system, but nonetheless, capable of causing the alarm to be operated.
A further object of the present invention is to provide burglar alarm systems wherein the unauthorized opening of an entranceway is signaled by mechanical transducers, thereby freeing the actual opening detection system from the need for electrical power, battery or otherwise.
Another and related object of the present invention is to provide one or more mechanical transducers for such burglar alarm systems which generate signals of predetermined frequency.
Yet another object of the present invention is to provide receivers, such as sonic receivers, equipped to receive only signals of the noted predetermined frequency and to reject all other signals.
Still another and related object of the present invention is to provide a superheterodyne sonic receiver with improved means for accepting and distinguishing desired signals which are used to enable a radio frequency generator, or other device as an integral part of the burglar alarm system. This receiver has means and circuitry of such a nature that prevents any spurious signals from activating the unit and causing false alarms.
Still another object of the present invention is to provide means for transmitting the radio frequency signals over power lines to a remote radio frequency receiver for operating alarm means.
Yet another object of the present invention is to provide means associated with the radio frequency generators and receivers for further protecting the system from spurious signals.